MAXI Detections of Superbursts– Updates on Long X-ray Bursts –Motoko Serino1 and Wataru Iwakiri11MAXI team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
E-mail(MS): motoko@crab.riken.jp
Abstract

In seven years, MAXI observed twelve long X-ray bursts, which were observed during at least twoconsecutive scans. We divided them into two classes according to their e-folding decay time: the bursts
with longer e-folding decay time (> 1 hour) are superbursts, and the others are intermediate duration
bursts. MAXI detected superbursts from two bright persistent source (Ser X-1 and 4U 1705−44), a dimpersistent source (4U 0614+091), and four transient sources (Aql X-1, SAX J1747.0−2853, EXO 1745−248,and SAX J1828.5−1037). Two of the superbursts from transient sources occurred at the time of very low
accretion rate, which may be before the outbursts. Four out of five intermediate duration bursts had low(< 2% of the Eddington limit) persistent flux. Four intermediate duration bursts are from ultracompact
X-ray binary. These characteristics are common to the intermediate duration bursts which were observed
before the MAXI era. We found possible anti-correlation between the decay time and the peak flux.
Superbursts have low peak fluxes (10–50% of the Eddington limit) and intermediate duration bursts havethe peak fluxes of about the Eddington limit.
Key words: stars: neutron — X-rays: bursts — MAXI

1. Introduction
MAXI scans about 85% of the whole sky in 92 minutes.
It is efficient for searching for transient events with long(>92 min) duration such as long X-ray bursts. As ofthe end of 2016, we found twelve long X-ray bursts in
the MAXI data. Table 1 shows the list of long X-raybursts. Eight of them overlap with the list in in ’t Zandet al. (2017). Most of them are described in the previous
papers (Serino et al. 2016; Serino et al. 2017). Three
new events were found after Serino et al. (2016), whichwere IGR J17062−6143 (Negoro et al. 2015; Iwakiri etal. 2015; Keek et al. 2017), Ser X-1 (Iwakiri et al. 2016),
and 4U 1705−44 (Iwakiri et al. 2016). Details of Ser X1 and 4U 1705−44 are also introduced by Iwakiri et al.
(2017) in this proceedings. In this paper, we focus on
the persistent flux of the long X-ray bursts and possible
anti-correlation between the decay time and the peakflux.
2. The persistent emissions of the long X-ray burst sources
MAXI monitors the persistent fluxes of X-ray burst
sources. Figure 1 shows an image of galactic sourcesin 4–10 keV. The previously known superburst sources(dashed circles) and the sources in Table 1 (solid circles)

Table 1. A list of long X-ray bursts observed by MAXI (sorted bydate)Name
4U 1820−30
SAX J1747.0−2853
EXO 1745−248
SAX J1828.5−1037
Ser X-1
SLX 1735−269
Aql X-1
4U 1850−086 (1)4U 0614+091
4U 1850−086 (2)IGR J17062−6143
4U 1705−44

date
2010-03-17
2011-02-13
2011-10-24
2011-11-12
2011-12-06
2012-12-06
2013-07-20
2014-03-10
2014-11-03
2015-05-09
2015-11-03
2016-10-22

decay time > 1h?
n
Y
Y
Y
Yn
Yn
Ynn
Y

10
Ser X-1
KS 1731-260

SLX 1735-269
EXO 1745-248
GX 3+1SAX J1747.0-2853

GX 17+2

4U 1608-52

4U 1705-440

SAX J1828.5-1037
4030 4U 1850-086 20Aql X-1

100350340

330320310300

H 1636-536
4U 1254-690

4U 1735-444-104U 1820-30

IGR J17062-6143

Fig. 1. A MAXI image of galactic sources in 4–10 keV, based on data collection in 4.7 years. The previously known superburst sources(dashed) and the sources in Table 1 (solid) are marked with circles.

0.3
0.2
0.1
0-0.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
04-10 keV-40-20020time since superburst [day]40-2 -1

2-4 keV

Flux [photons cm s ]2SAXJ1747

-2 -1

0.4

Flux [photons cm s ]-2 -1

Flux [photons cm s ]-2 -1

Flux [photons cm s ]0.5AqlX-1

2-4 keV

1.5
1
0.5
0-0.5
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0-0.2

4-10 keV-40-20020time since superburst [day]40Fig. 2. Light curves of the outbursts of SAX J1747.0−2853 (left) and Aql X-1 (right). A point correspond to a scan. The short flares attime= 0 are superbursts.

3. Peak flux – decay time correlation
Figure 3 shows a scatter plot of the peak flux and thee-folding decay time τ of long bursts. Since the peak
fluxes are normalized with the observed maximum fluxesof the normal bursts, which are an approximation of theEddington ratio2 . There are systematic uncertainties ofthe burst peak flux, because of the limited scan time
window. Then we estimate the “possible maxima” by*1 http://maxi.riken.jp/top/*2 We do not have information of a normal X-ray burst fromIGR J17062−6143. We assume the observed peak flux of thelong burst in our sample was the Eddington limit, because theinferred radius suggested a photospheric expansion (Keek etal. 2017).

extrapolating the exponential function to the epoch of
the previous scan transit. The upper ends of the horizontal bars in figure 3 are these “possible maximum”.
There may be an anti-correlation between the decay
time and the peak flux. Superbursts have low peak fluxes(10–50% of the Eddington limit) and intermediate duration bursts have the peak fluxes of about the Eddington
limit.
The authors thank Jean in ’t Zand for helpful comments on this paper and continuous collaboration withus.
104U0614

AqlX-1

EXO1745SAXJ1747

SerX-1
SAXJ1828

τ (hour)are marked. The former tend to be brighter than thelatter.
Two superbursts from SAX J1747.0−2853 and Aql X1 occurred during transient outbursts. The outburstsof these sources started ∼ 25 and ∼ 40 days before thesuperbursts. They are similar to 4U 1608−522, which
also occurred during the outburst and it started 57.6
days before the superburst (Keek et al. 2008).
No superburst was found from bright and persistentsources in the MAXI data before 2016. Serino et al.
(2016) mentioned that it should be a selection bias, because it is easier to find superbursts from sources withweaker persistent emissions. Then we searched for superbursts from bright sources carefully, and found a superburst from Ser X-1. Another superburst, which wasobserved just before ”7 years of MAXI” conference, wasalso from a bright persistent source 4U 1705−44 (Iwakiriet al. 2016; Iwakiri et al. 2017).
The MAXI light curve1 of SAX J1828.5−1037 stays
constant at ∼ 10 mCrab. We note that this emission
is dominated by the galactic ridge emission (see figure1). The light curves of the sources near the galactic center region (SLX 1735−269, SAX J1747.0−2853,and EXO 1745−248) are also contaminated by nearby
sources. Therefore it is difficult to estimate the persistent flux from these sources.
We classified five bursts into the intermediate duration bursts, because they have exponential decay time
below 1 hour. Four out of these five had low (< 2% ofthe Eddington limit) persistent flux. Three sources (4U1820−30, 4U 1850−086, and SLX 1735−269) with theintermediate duration bursts are (candidate) ultracompact X-ray binaries, while the compactness of the othersource (IGR J17062−6143) is not known. Ultracompact
nature and low persistent luminosity are common to thesources of intermediate duration bursts (Falanga et al.
2008). The persistent emission of 4U 1820−30 is exceptionally bright unlike other sources of intermediate
duration bursts.

4U1705

1SLX1735
4U1850(2nd)

4U1820

IGRJ17062
4U1850(1st)0.1
101001000

Peak flux normalized with maximum flux of normal bursts (%)

Fig. 3. A scatter plot of peak flux and e-folding time of long bursts.
Each peak flux in the plot are normalized with the maximum
flux of the normal bursts of the same source. Three new events
discovered after Serino et al. (2016) are labeled with larger font.
Note that the horizontal bars are not statistic errors. The lower
ends of the fluxes are the observed ones, and the upper ends are
possible maximum fluxes (see text).

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